Nuclear magnetic resonance imaging (MRI) serves as an indispensable tool for obtaining images of living specimens such as human beings or single cells in a completely non-invasive manner and without the use of ionizing radiation. In addition, MRI is unique in allowing the generation of images with various kinds of contrast, and therefore images with different information content. MRI is also becoming increasingly important in materials sciences, where the non-destructive nature of the method is of particular interest.
The conventional MRI method relies on the use of two magnetic fields, which are generated by two coils or coil arrays: One is a homogeneous field (called the “B0 field”), the other a supplementary field of known inhomogeneity (called the “gradient field”). The latter field defines the obtainable spatial resolution of the experiment. Generally, finer details of the subject or object under investigation can be revealed if the gradient field is made stronger. However, the maximum allowed strength of the gradient field is limited for the MRI methods practiced today. When the gradient field strength becomes comparable with that of the homogeneous B0 field or even stronger, the conventional methods of image formation start to fail.
When operating at high homogeneous fields of 1.5 or 3 Tesla for example, the condition that the gradient field strength has to be considerably weaker than the B0 field is often readily satisfied. However, even in this situation small but noticeable artifacts can become apparent that have to be corrected in post processing. However, the restricted maximum gradient field strength severely limits imaging in situations where the B0 field is not large compared to the gradient field.
Recent attempts have been made to reduce the values of the required magnetic fields in MRI imaging applications. In recent years, low field setups have been shown to be capable of producing images of good quality within useful (in the context of human imaging) periods of time. In addition, low field MRI is insensitive to certain classes of artifacts such as ghosting due to susceptibility broadening or chemical shift. The latter are omnipresent in high field MRI and, for example, make the imaging of heterogeneous samples difficult or sometimes even impossible. However, the fact that the gradient field still has to be significantly smaller than the weak B0 field imposes limitations to the obtainable spatial resolution of the images.
The physical reason why the homogeneous field has to be considerably (typically five to ten times) stronger than the maximum gradient field is the following: As longs as the gradient field is applied in the presence of a stronger (typically homogeneous) field, certain components of the gradient field become negligible, the field is said to be “truncated”. All of the practical MRI schemes in use today rely on truncated gradient fields. When the B0 field is not strong enough to truncate the gradient field, the complete field generated by the gradient field coils has to be considered. Conventional approaches to image formation fail in these circumstances.